This invention relates to microelectronic manufacturing methods and devices, and more particularly to silicon ingot manufacturing methods and silicon ingots and wafers manufactured thereby.
Integrated circuits are widely used in consumer and commercial applications. Integrated circuits are generally fabricated from monocrystalline silicon. As the integration density of integrated circuits continues to increase, it generally is of increasing importance to provide high-quality monocrystalline semiconductor material for integrated circuits. Integrated circuits are typically produced by fabricating a large ingot of monocrystalline silicon, slicing the ingot into wafers, performing numerous microelectronic fabrication processes on the wafers and then dicing the wafers into individual integrated circuits that are packaged. Because the purity and crystallinity of the silicon ingot can have a large impact on the performance of the ultimate integrated circuit devices that are fabricated therefrom, increased efforts have been made to fabricate ingots and wafers with reduced numbers of defects.
Conventional methods of manufacturing monocrystalline silicon ingots will now be described. An overview of these methods is provided in Chapter 1 of the textbook xe2x80x9cSilicon Processing for the VLSI Era, Volume 1, Process Technologyxe2x80x9d, by Wolf and Tauber, 1986, pp. 1-35, the disclosure of which is hereby incorporated herein by reference. In manufacturing monocrystalline silicon, electronic grade polysilicon is converted into a monocrystalline silicon ingot. Polycrystalline silicon such as quartzite is refined to produce electronic grade polycrystalline silicon. The refined electronic grade polycrystalline silicon is then grown into a single crystal ingot using the Czochralski (CZ) or Float Zone (FZ) technique. Since the present invention relates to manufacturing a silicon ingot using the CZ technique, this technique will now be described.
Czochralski growth involves crystalline solidification of atoms from a liquid phase at an interface. In particular, a crucible is loaded with a charge of electronic grade polycrystalline silicon and the charge is melted. A seed crystal of silicon of precise orientation tolerances is lowered into the silicon melt. The seed crystal is then withdrawn at a controlled rate in the axial direction. Both the seed crystal and the crucible are generally rotated during the pulling process, in opposite directions.
The initial pull rate is generally relatively rapid so that a thin neck of silicon is produced. Then, the melt temperature is reduced and stabilized so that the desired ingot diameter can be formed. This diameter is generally maintained by controlling the pull rate. The pulling continues until the melt is nearly exhausted, at which time a tail is formed.
FIG. 1 is a schematic diagram of a Czochralski puller. As shown in FIG. 1, the Czochralski puller 100 includes a furnace, a crystal pulling mechanism, an environment controller and a computer-based control system. The Czochralski furnace is generally referred to as a hot zone furnace. The hot zone furnace includes a heater 104, a crucible 106 which may be made of quartz, a succeptor 108 which may be made of graphite and a rotation shaft 110 that rotates about an axis in a first direction 112 as shown.
A cooling jacket or port 132 is cooled by external cooling means such as water cooling. A heat shield 114 may provide additional thermal distribution. A heat pack 102 is filled with heat absorbing material 116 to provide additional thermal distribution.
The crystal pulling mechanism includes a crystal pulling shaft 120 which may rotate about the axis in a direction 122 opposite the direction 112 as shown. The crystal pulling shaft 120 includes a seed holder 120a at the end thereof. The seed holder 120a holds a seed crystal 124, which is pulled from the melt 126 in the crucible 106 to form an ingot 128.
The ambient control system may include the chamber enclosure 140, the cooling jacket 132 and other flow controllers and vacuum exhaust systems that are not shown. A computer-based control system may be used to control the heating elements, puller and other electrical and mechanical elements.
In order to grow a monocrystalline silicon ingot, the seed crystal 124 is contacted to the silicon melt 126 and is gradually pulled in the axial direction (up). Cooling and solidification of the silicon melt 126 into monocrystalline silicon occurs at the interface 130 between the ingot 128 and the melt 126. As shown in FIG. 1, the interface 130 is concave relative to the melt 126.
Real silicon ingots differ from ideal monocrystalline ingots because they include imperfections or defects. These defects are undesirable in fabricating integrated circuit devices. These defects may be generally classified as point defects or agglomerates (three-dimensional defects). Point defects are of two general types: vacancy point defects and interstitial point defects. In a vacancy point defect, a silicon atom is missing from one of its normal positions in the silicon crystal lattice. This vacancy gives rise to a vacancy point defect. On the other hand, if an atom is found at a non-lattice site (interstitial site) in the silicon crystal, it gives rise to an interstitial point defect.
Point defects are generally formed at the interface 130 between the silicon melt 126 and the solid silicon 128. However, as the ingot 128 continues to be pulled, the portion that was at the interface begins to cool. During cooling, diffusion of vacancy point defects and interstitial point defects may cause defects to coalesce and form vacancy agglomerates or interstitial agglomerates. Agglomerates are three-dimensional (large) structures that arise due to coalescence of point defects. Interstitial agglomerates are also referred to as dislocation defects or D-defects. Agglomerates are also sometimes named by the technique that is used to detect these defects. Thus, vacancy agglomerates are sometimes referred to as Crystal-Originated Particles (COP), Laser Scattering Tomography (LST) defects or Flow Pattern Defects (FPD). Interstitial agglomerates are also known as Large Dislocation (L/D) agglomerates. A discussion of defects in monocrystalline silicon is provided in Chapter 2 of the above-mentioned textbook by Wolf and Tauber, the disclosure of which is hereby incorporated herein by reference.
It is known that many parameters may need to be controlled in order to grow a high purity ingot having low numbers of defects. For example, it is known to control the pull rate of the seed crystal and the temperature gradients in the hot zone structure. Voronkov""s Theory found that the ratio of V to G (referred to as V/G) can determine the point defect concentration in the ingot, where V is the pull rate of the ingot and G is the temperature gradient of the ingot-melt interface. Voronkov""s Theory is described in detail in xe2x80x9cThe Mechanism of Swirl Defects Formation in Siliconxe2x80x9d by Voronkov, Journal of Crystal Growth, Vol. 59, 1982, pp. 625-643.
An application of Voronkov""s Theory may be found in a publication by the present inventor et al. entitled xe2x80x9cEffect of Crystal Defects on Device Characteristicsxe2x80x9d, Proceedings of the Second International Symposium on Advanced Science and Technology of Silicon Material, November 25-29, 1996, p. 519. At FIG. 15, reproduced herein as FIG. 2, a graphical illustration of vacancy and interstitial concentrations, as a function of V/G, is shown. Voronkov""s Theory shows that the generation of a vacancy/interstitial mixture in a wafer is determined by V/G. More particularly, for V/G ratios below a critical ratio, an interstitial rich ingot is formed, while for V/G ratios above the critical ratio, a vacancy rich ingot is formed.
Notwithstanding many theoretical investigations by physicists, material scientists and others, and many practical investigations by Czochralski puller manufacturers, there continues to be a need to provide Czochralski pullers that can reduce the defect density in monocrystalline silicon wafers.
The present invention provides heat shields for Czochralski pullers that include a ring-shaped heat shield housing comprising inner and outer heat shield housing walls and an oblique heat shield housing floor and a heat shield housing roof that extend between the inner and outer heat shield housing walls. The heat shield housing contains insulating material therein. A support member is configured to support the heat shield housing within the crucible in a Czochralski puller. The inner and outer heat shield walls preferably are vertical inner and outer heat shield walls, and the heat shield housing roof preferably is an oblique heat shield housing roof.
In one embodiment, the support member includes at least one support arm that extends to the ring-shaped heat shield housing. The at least one support arm may be hollow and may contain insulating material therein. In another embodiment, the support member is a ring-shaped support member. The ring-shaped support member may include inner and outer support member walls containing insulating material therebetween. The ring-shaped support member may also include at least one window therein. The ring-shaped member may be oblique.
Czochralski pullers according to the present invention may include an enclosure, a crucible in the enclosure that holds a silicon melt, a seed holder in the enclosure adjacent the crucible and a heater in the enclosure surrounding the crucible. A heat shield as described above may also be provided, including a ring-shaped heat shield housing within the crucible and a support member that supports the heat shield housing within the crucible. Czochralski pullers also include means for pulling the seed holder away from the crucible, to thereby pull a monocrystalline silicon ingot from the silicon melt. The monocrystalline silicon ingot has an axis and a cylindrical edge. The silicon melt and the ingot define an ingot-melt interface therebetween. The oblique heat shield floor makes a first angle with the horizontal and the oblique heat shield housing roof makes a second angle with the horizontal. At least one of the inner wall length, the first angle and the second angle preferably are selected to produce a temperature gradient at the ingot-melt interface at the axis that is at least about equal to the temperature gradient at a diffusion length from the cylindrical edge.
According to another aspect of the invention, the Czochralski puller also includes a heat pack in the enclosure, surrounding the heater. The heat pack includes an upper heat pack housing and a lower heat pack housing. The lower heat pack housing is filled with heat absorbing material. However, the upper heat pack housing is at least partially unfilled with the heat absorbing material. Preferably, all of the heat absorbing material is removed from the upper heat pack so that the upper heat pack housing is free of the heat absorbing material.
The heat shield support member is preferably attached to the upper heat pack housing to support the ring-shaped heat shield housing within the crucible. Accordingly, improved heat shields and Czochralski pullers may be provided.